How to create a watermark
Add a new static slide
After you created the slider where you would like to have the watermark, add a new static slide by clicking on the small arrow at the end of the "Add image" button.
Static slides will be over all of your slides, so you don't have to add the watermark on all of the images.
Add the watermark as a layer
Add a new image layer select your image and move it to the position where you would like to have it. Save the slide.
That's it, if you preview your slider you'll see the watermark on all of the slides.